Discovery of five very low mass close binaries, resolved in the visible with lucky imaging
We survey a sample of 32 M5–M8 stars with distance <40 pc for companions with separations between 0.1 and 1.5 arcsec and with Δmi≤ 5. We find five new binaries with separations between 0.15 and 1.1 arcsec, including a candidate brown dwarf companion. The raw binary fraction is 16+8−4 per cent and the distance bias corrected fraction is 7+7−3 per cent, for companions within the surveyed range. No systems with contrast ratio Δmi > 1 were found, even though our survey is sensitive to Δm≤ 5 (well into the brown dwarf regime). The distribution of orbital radii is in broad agreement with previous results, with most systems at 1–5 au, but one detected binary is very wide at 46.8 ± 5.0 au. We also serendipitously imaged for the first time a companion to Ross 530, a metal-poor single-lined spectroscopic binary. We used the new Lucky Imaging system, LuckyCam, on the 2.5-m Nordic Optical Telescope to complete the 32 very low mass star Sloan Digital Sky Survey (SDSS) i′ and z′ survey in only 5 h of telescope time.